1. Field of the Invention
The present invention relates to the preparation of metal oxides and, more particularly, to the preparation of particulate metal oxides of extremely small particle size.
2. Description of the Background
Ceramics have found widespread use in virtually all phases of modern technology. They find use as abrasives, in making cutting tools, as heat shields and electrical insulators and in such high tech uses as computers and telecommunications. They are virtually impervious to attack by heat or chemicals and can be made in a wide variety of forms depending on their intended end use. Although ceramics are virtually unparalleled in their ability to withstand heat and chemical attack, structurally, they suffer from the fact that they are brittle with the result that they are easily susceptible to cracking and breaking. It is known that imperfections in the microstructure of ceramics serve as starting points for cracks. Accordingly, as new high tech ceramics are developed, there is increasing emphasis to develop processing techniques and material for forming ceramics which will minimize the imperfections in the microstructure.
It is known that the minute imperfections leading to cracks in ceramics can be minimized if the raw material powder used to form the ceramic is of very high purity and extremely small particle size. High purity, fine powders permit tight packing meaning that the sintered product will have less voids, agglomerations and chemical impurities, all of which can form sites from which cracks can originate.
Metal oxide powders, particularly alumina, also find widespread use as catalysts and catalyst supports in the chemical and refining industries.
Conventional methods of preparing small particle size powders, such as, for example, alumina, include the grinding of larger particles to the desired size or forming the particle in the vapor phase by so-called fuming techniques. Additionally, as taught in "Chemistry's New Workhorse", High Technology July 1987, small particle sizes of high purity can be obtained by sol-gel techniques wherein the very fine particles of the metal oxide are precipitated from a liquid phase.
In a process described in "Preparation of Very Pure, Finely Divided Aluminum Oxide From Triethyl Aluminum" by K. H. Thiele, W. Schwartz and L. Dettmann, Z. Anorg. Allgem. Chem., 349; 324-327 (967), triethyl aluminum is conducted into an oxyhydrogen flame to convert the triethyl alumina to particles of aluminum oxide. This procedure requires that the triethyl aluminum be vaporized and mixed with a hydrogen stream.
U.K. Patent Application 2,166,126A discloses a process for producing ceramic powders in which at least one substance which will hydrolyze to produce the metallic or non-metallic oxide, hydroxide or hydrate is sprayed and hydrolyzed, in the presence of water, the hydrolyzed material then being introduced into a calcining zone to calcine the hydrolysis product.
An article entitled "Fine Si.sub.3 N.sub.4 and SiC Powders Prepared by Vapor Phase Hydrolysis" presented at the Conference on Ceramic Powder Science and Technology, Boston, Mass., Aug. 3-6, 1986, discloses the preparation of silicon nitride and silicon carbide powders by the vapor-phase pyrolysis of tris (dimethylamino) silane in an inductively heated alumina tube reactor.